Method of regenerating NOx removal catalyst and method of operating heavy-oil fired combustion apparatus

ABSTRACT

The method includes a pretreatment step during an operation of a boiler in which in a predetermined period of time before shutdown of the boiler, a part of combustion gas that has bypassed an economizer provided in a flue gas duct for flue gas from the boiler is supplied to an upstream of a NO x  removal device having a NO x  removal catalyst and mixed with the combustion flue gas from the economizer to generate mixed gas having a predetermined temperature equal to or higher than 360° C. (360° C. to 450° C.), the mixed gas is introduced into the NO x  removal catalyst, thereby decomposing VOSO 4  adhering to and accumulating on the NO x  removal catalyst into V 2 O 5 .

FIELD

The present invention relates to a method of regenerating a NO_(x)removal catalyst of a NO_(x) removal device that performs flue-gastreatment in a heavy-oil fired combustion apparatus and a method ofoperating a heavy-oil fired combustion apparatus.

BACKGROUND

As a method of removing nitrogen oxide (NO_(x)) in flue gas dischargedfrom a combustion apparatus such as a boiler, an ammonia catalyticreduction method has been put into practical use, in which ammonia (NH₃)is used as a reduction agent to decompose NO_(x) to nitrogen and water,which are harmless to the environment, in the presence of a nitrogenoxide removal catalyst (hereinafter, “NO_(x) removal catalyst”).

In the treatment of flue gas from a boiler that uses heavy oil such asfuel oil C with a high sulfur (S) content as a fuel, sulfur dioxide(SO₂) with a high concentration is present in flue gas. Therefore,oxidation reaction of sulfur dioxide (SO₂) into sulfur trioxide (SO₃),which occurs simultaneously with reduction and removal reaction of aNO_(x) removal catalyst, produces sulfur trioxide (SO₃) with a highconcentration. The sulfur trioxide (SO₃) with a high concentration iseasily bound to an unreacted portion of NH₃, which is used as areduction agent, in a low temperature region to form acid ammoniumsulfate and other compounds, and thus clogging or partial blockages tothe interior and piping of various devices such as a heat exchanger in adownstream or the like, thereby increasing a pressure loss. Accordingly,a measure such as performance upgrading of a precipitator needs to betaken.

Furthermore, as a catalyst exhibiting an excellent NO_(x) removalperformance and a low SO₂ oxidation capability, a type of catalysthaving tungsten oxide, vanadium-tungsten oxide or the like supported ontitania is available.

Ultraheavy oil, such as Orimulsion (for an oil-in-water type emulsionproduced by mixing Orinoco tar, which is ultraheavy oil collected inOrinoco State, Venezuela, with water and a surfactant for easy handlingat a normal temperature; trade name of Mitsubishi Corporation), asphalt,and vacuum residual oil (VRO), contains two to three times as muchsulfur (S), and five to seven times as much vanadium (V) as in fuel oilC, as shown in Table 1 disclosed in the Patent Literature mentionedbelow. Furthermore, because the SO₂ concentration in flue gas of suchultraheavy oil is very high, there is a problem that a vanadium compoundbecomes deposited on a surface of the NO_(x) removal catalyst during anoperation.

Particularly, it has been confirmed that when a low grade fuel with asulfur (S) content of 1% by weight or more is used, this tendency isnoticeable.

TABLE 1 Measurement item Orimulsion Orinoco oil Fuel oil C Specificgravity — 0.976  0.889 (80° C./4° C.) Viscosity — 1528 34.8  (cSt) (80°C.) N (wt %) 0.48 0.63 0.22 S (wt %) 2.78 3.79 0.97 Water content (wt %)28 to 30 <0.1 <0.1  Higher calorific power About 29400 About 4200043680     (kJ/kg) [kcal/kg] [about 7000] [about 10000] [10400]     Ashcontent (wt %) 0.08 0.13 0.01 Carbon residue (wt %) 11.89 17.4 8.0  V(ppm) 280 to 350 400 to 500 50 or less Na (ppm) 40 to 70  60 to 100 —(Reference) The Thermal and Nuclear Power, Vol. 465, June 1995

FIG. 5 is a graph of an X-ray diffraction analysis result of a NO_(x)removal catalyst after being operated for 14,400 hours. FIG. 6 is agraph of a relation between a boiler operating time and an increasedamount of vanadium pentoxide (V₂O₅) in an entire catalyst. FIG. 7 is agraph of a relation between a concentration increase of vanadiumpentoxide (V₂O₅) and an SO₂ oxidation rate (Non Patent Literature 1).Here, “a.u.” in FIGS. 5, 6, and 7 represents a ratio to a referencevalue, and is not an absolute value.

As shown in FIG. 5, in the X-ray diffraction analysis result in aninitial stage of an operation, only titania (TiO) of the catalystcomponent was produced. However, in the X-ray diffraction analysisresult after being operated for 14,400 hours, it was confirmed thatvanadyl sulfate (VOSO₄) and vanadium pentoxide (V₂O₅) were produced inthe NO_(x) removal catalyst. As shown in FIG. 6, it was confirmed thatthe concentration of vanadium pentoxide (V₂O₅) is increased over thecourse of operating hours.

The vanadium is an active component of the NO_(x) removal catalyst andis a substance that promotes SO₂ oxidation reaction as a side reaction.As shown in FIG. 7, in an actual device, there is such a problem thatthe SO₂ oxidation reaction rate is increased with time, and as a result,the amount of corrosive SO₃ discharged to a downstream side of theNO_(x) removal device is increased.

In catalyst regeneration treatment for regenerating a NO_(x) removalcatalyst at the time of an inspection, therefore, the present inventorshave proposed a technique for regenerating a NO_(x) removal catalyst inwhich after the NO_(x) removal catalyst is first taken out from a NO_(x)removal device, the NO_(x) removal catalyst is then heated under a hightemperature condition of, for example, from 450° C. to 600° C. in anelectric furnace, vanadyl sulfate (VOSO₄) is decomposed into vanadiumpentoxide (V₂O₅), and thereafter the heated NO_(x) removal catalyst issubjected to oxalic acid washing using oxalic acid, thereby dissolvingand removing vanadium pentoxide (V₂O₅), and a technique for decomposingvanadyl sulfate (VOSO₄) under a high temperature condition by usingsulfuric acid or the like (Patent Literature 1).

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Application Laid-open No.    2005-185928

Non Patent Literature

-   Non Patent Literature 1: Mitsubishi Heavy Industries Technical    Review Vol. 38, No. 1 (2001-1)

SUMMARY Technical Problem

However, in the method of regenerating a NO_(x) removal catalystdisclosed in Patent Literature 1, at the time of heating the NO_(x)removal catalyst by using the electric furnace under a high temperaturecondition (for example, from 450° C. to 600° C.), there is a problemthat a reaction in the following formula (1) proceeds, and sulfurdioxide (SO₂) and sulfur trioxide (SO₃) are produced in the electricfurnace.2VOSO₄→V₂O₅+SO₂+SO₃  (1)

Therefore, in the catalyst regeneration method according to PatentLiterature 1, at the time of performing high-temperature heatingtreatment in the electric furnace, there is a problem that adesulfurizer is separately required as a treatment facility of sulfuroxide (SO_(x)).

Further, to decompose vanadyl sulfate (VOSO₄) only by acid treatment, itis required to set the acid concentration to 3 to 12 Normal and toperform the treatment under a severe condition such as a temperature of60° C. to 90° C., and thus the environment of the treatment facilitybecomes degraded.

Therefore, the emergence of a method of regenerating a NO_(x) removalcatalyst that can facilitate regeneration of the NO_(x) removal catalysthas been strongly desired.

Solution to Problem

The present invention has been achieved to solve the above problems, andan object of the present invention is to provide a method ofregenerating a NO_(x) removal catalyst and a method of operating aheavy-oil fired combustion apparatus.

Solution to Problem

According to an aspect of the present invention, in a method ofregenerating a NO_(x) removal catalyst for removing nitrogen oxide influe gas at a time of combustion in a boiler by using heavy oil, in apredetermined period of time before shutdown of a boiler, a decomposingstep is performed during an operation of the boiler, at which a part ofcombustion gas that has bypassed an economizer provided in a flue gasduct for flue gas from the boiler is supplied to an upstream of a NO_(x)removal device having a NO_(x) removal catalyst and mixed with flue gasfrom the economizer, a temperature of mixed gas is increased to apredetermined temperature equal to or higher than 360° C. to have theNO_(x) removal device in a high-temperature denitration condition, sothat vanadyl sulfate (VOSO₄) adhering to and accumulating on the NO_(x)removal catalyst is decomposed into vanadium pentoxide (V₂O₅).

Advantageously, the method of regenerating a NO_(x) removal catalystincludes an acid washing step after the decomposing step, at which anoperation of the boiler is stopped, and thereafter the NO_(x) removalcatalyst taken out from the NO_(x) removal device is subjected to acidwashing.

Advantageously, in the method of regenerating a NO_(x) removal catalyst,in a boiler operation period at the decomposing step, fuel is changed tooil fuel with a low sulfur (S) content and is burned in the boiler.

Advantageously, in the method of regenerating a NO_(x) removal catalyst,acid used at the acid washing step is any aqueous solution of oxalicacid, hydrochloric acid, and sulfuric acid.

Advantageously, the method of regenerating a NO_(x) removal catalystincludes a water washing step of performing finish water washing and adrying step after the acid washing step.

Advantageously, in the method of regenerating a NO_(x) removal catalyst,oil fuel with a low sulfur (S) content is oil fuel that contains 1% byweight or less of sulfur (S).

According to another aspect of the present invention, in a method ofoperating a heavy-oil fired combustion apparatus that burns heavy oil ina boiler, at an operating step before shutdown of a boiler, a part ofcombustion gas that has bypassed an economizer provided in a flue gasduct for flue gas from the boiler is supplied to an upstream of a NO_(x)removal device and mixed with flue gas from the economizer, atemperature of mixed gas is increased to a predetermined temperatureequal to or higher than 360° C., which is higher than a normal operatingtemperature condition, to have the NO_(x) removal device in ahigh-temperature denitration condition, so that nitrogen oxide in fluegas is removed during an operation of the NO_(x) removal device, andvanadyl sulfate (VOSO₄) adhered and accumulated on a NO_(x) removalcatalyst is decomposed into vanadium pentoxide (V₂O₅), and sulfur oxide(SO_(x)) produced at a time of decomposing vanadyl sulfate (VOSO₄) intovanadium pentoxide (V₂O₅) at the operating step before shutdown istreated in an air pollution control facility on a downstream side of theNO_(x) removal device.

Advantageously, in the method of operating a heavy-oil fired combustionapparatus, at a time of a normal operation, combustion is performed inthe boiler by using heavy oil with a high sulfur (S) content, and beforepredetermined shutdown for a boiler inspection, fuel is changed to oilfuel with a low sulfur (S) content and burned in the boiler.

Advantageously, in the method of operating a heavy-oil fired combustionapparatus, a temperature of flue gas from an economizer outlet at anormal operating step is set equal to or higher than 360° C., and atemperature of the mixed gas is set equal to or higher than 450° C.

Advantageously, in the method of operating a heavy-oil fired combustionapparatus, the oil fuel with a low sulfur (S) content is oil fuel thatcontains 1% by weight or less of sulfur (S).

Advantageous Effects of Invention

According to the present invention, at the time of recovering a NO_(x)removal performance of a NO_(x) removal catalyst used for removingNO_(x) from flue gas discharged from a boiler in which combustion isperformed by using heavy oil with a high sulfur (S) content, removal ofNO_(x) can be easily performed by using an air pollution control deviceof a boiler facility.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of an air pollution control device of aheavy-oil fired boiler.

FIG. 2 is a schematic diagram of a heat exchanger including aneconomizer and a NO_(x) removal device.

FIG. 3 depicts an X-ray diffraction pattern before heating andregeneration.

FIG. 4 depicts an X-ray diffraction pattern after performing heatingtreatment with gas containing SO₂ and SO₃.

FIG. 5 is a graph of an X-ray diffraction analysis result of a NO_(x)removal catalyst after being operated for 14,400 hours.

FIG. 6 is a graph of a relation between a boiler operating time and anincreased amount of vanadium pentoxide (V₂O₅) in an entire catalyst.

FIG. 7 is a graph of a relation between a concentration increase ofvanadium pentoxide (V₂O₅) and an SO₂ oxidation rate.

DESCRIPTION OF EMBODIMENTS

The present invention will be explained below in detail with referenceto the accompanying drawings. The present invention is not limited thefollowing embodiments, and configurations achieved by combining theseembodiments are also included in the present invention. In addition,constituent elements in the following embodiments include those that canbe easily anticipated by persons skilled in the art or that aresubstantially equivalent.

Embodiment

A method of regenerating a NO_(x) removal catalyst and a method ofoperating a heavy-oil fired combustion apparatus according to anembodiment of the present invention will be explained below withreference to the drawings.

FIG. 1 is a schematic diagram of an air pollution control device of aheavy-oil fired boiler. As shown in FIG. 1, combustion gas 11 in aheavy-oil fired boiler 10 that uses heavy oil F generates steam in asteam generating tube in a furnace 12 (the generated steam isvapor-liquid separated in a steam drum 13, guided to a super heater 14to become superheated steam, which is then used for driving a steamturbine, and thereafter condensed water is refluxed to a water tube inthe furnace 12 and re-evaporated), and the steam is heated by the superheater 14. The combustion gas 11 then heats feed water to the heavy-oilfired boiler 10 in an economizer 15, and is discharged from an outlet ofthe economizer 15 as flue gas 16. The flue gas 16 discharged from theeconomizer 15 is supplied to a NO_(x) removal device 17 that removesnitrogen oxide (NO_(x)) in flue gas, heats air 19 by heat exchange in anair heater 18, and is then supplied to a precipitator 20. The flue gas16 is also supplied to a desulfurizer 21 that desulfurizes sulfur oxide(SO_(x)) in the flue gas, and then discharged to the atmosphere aspurified gas 22.

As the NO_(x) removal device 17, a device that performs catalyticreduction of NO_(x) in the flue gas 16 from the heavy-oil fired boiler10 by spraying ammonium (NH₃) to a downstream side of the NO_(x) removaldevice (a catalytic portion) 17 has been proposed.

FIG. 2 is a schematic diagram of a heat exchanger including theeconomizer and the NO_(x) removal device.

As shown in FIG. 2, the economizer 15 is provided in the last stage of aheat exchanger 10 a, and an economizer bypass 15 a for bypassing theeconomizer 15 is connected to a flue gas duct 23 for discharging theflue gas 16. A part 11 a of combustion gas flowing into the economizerbypass 15 a is mixed with the flue gas 16 discharged from the economizer15, which can be supplied as mixed gas 24 to the NO_(x) removal device17 including a NO_(x) removal catalyst 17 a. On an upstream side of theNO_(x) removal device 17, ammonia is sprayed to perform denitration byammonia.

Because the part 11 a of combustion gas bypassing the economizer 15through the economizer bypass 15 a is not used for heat exchange in theeconomizer 15, a gas temperature (T₂) thereof is as high as, forexample, 550° C. to 600° C., although depending on an operatingcondition of the boiler.

At the time of a normal operation, a part of flue gas can be set not toflow into the economizer bypass 15 a by a shutoff valve (not shown).

A temperature (T₁) of the flue gas 16 at the outlet of the economizer 15is about 350° C. to 360° C. because heat exchange is performed.

Accordingly, a temperature (T₃) of the mixed gas 24 in which the fluegas 16 at the outlet of the economizer 15 having a gas temperature (T₁)and a part of bypassed combustion gas having the gas temperature (T₂)are mixed can be equal to or higher than 360° C., which is higher thanthe temperature (T₁) of the flue gas 16 at the outlet of the economizer15, by adjusting a bypassed amount of the part 11 a of combustion gas.

Meanwhile, when heavy oil is burned in the boiler as described above,because a large amount of vanadium component is contained in the heavyoil, the vanadium component approaches the NO_(x) removal catalyst 17 aused in the NO_(x) removal device 17 of the air pollution controldevice, in a state of vanadium pentoxide (V₂O₅). However, once adheringto the NO_(x) removal catalyst 17 a, the vanadium component reacts withsulfur dioxide (SO₂) and sulfur trioxide (SO₃) contained in the flue gas16 as shown in the following reaction formula (2), and accumulates onthe NO_(x) removal catalyst 17 a in a state of vanadyl sulfate (VOSO₄).Accumulation of vanadyl sulfate (VOSO₄) becomes noticeable, when acontained amount of sulfur (S) is 1% by weight or more, althoughdepending on a contained amount of sulfur (S) in heavy oil.V₂O₅+SO₂+SO₃→2VOSO₄  (2)<Method of Regenerating NO_(x) Removal Catalyst>

The present invention provides a method of regenerating a NO_(x) removalcatalyst in which vanadyl sulfate (VOSO₄) accumulating on a surface ofthe NO_(x) removal catalyst is removed and regenerated, at the time ofburning heavy oil in a boiler.

The method of regenerating the NO_(x) removal catalyst according to thepresent invention includes the following processes.

1) Decomposing Process of VOSO₄ During Boiler Operation

In a pretreatment process during an operation of the boiler, the part 11a of the flue gas 16 from the boiler 10, which has bypassed theeconomizer provided in the flue gas duct 23, is supplied to an upstreamof the NO_(x) removal device including a NO_(x) removal catalyst, and ismixed with flue gas from the economizer in a predetermined period oftime before shutdown. The temperature of the mixed gas (T₃) is increasedto a predetermined temperature equal to or higher than 360° C. (360° C.to 450° C.) to have the NO_(x) removal device in a high-temperaturedenitration condition, so that vanadyl sulfate (VOSO₄) adhering to andaccumulating on the NO_(x) removal catalyst is decomposed into vanadiumpentoxide (V₂O₅).

1) In the pretreatment process during a boiler operation, vanadylsulfate (VOSO₄) adhering to and accumulating on the NO_(x) removalcatalyst is decomposed into vanadium pentoxide (V₂O₅), and the state ofa vanadium component remaining in the NO_(x) removal catalyst is changedto vanadium pentoxide (V₂O₅), which can be removed by washing withoxalic acid.

Therefore, in the present embodiment, after oil is changed from normalheavy oil with a high sulfur (S) content to oil fuel with a low sulfur(S) content, the part 11 a of combustion gas is introduced into theeconomizer bypass 15 a. However, the kind of fuel can be changed afterthe part 11 a of combustion gas is introduced.

In a predetermined period of time before shutdown of the boiler,combustion in the boiler can be performed by changing boiler fuel from atype of fuel having a high sulfur (S) content to oil fuel having alow-sulfur (S) content. Accordingly, decomposition of vanadyl sulfate(VOSO₄) can be promoted.

2) Acid Washing Process

After finishing the pretreatment process during the boiler operation in1), the boiler is shutdown, the NO_(x) removal catalyst 17 a is takenout from the NO_(x) removal device 17, the taken out NO_(x) removalcatalyst 17 a is subjected to acid washing treatment, thereby dissolvingand removing vanadium pentoxide (V₂O₅), which is the remaining vanadiumcomponent.

As an acid used for dissolving and removing vanadium pentoxide (V₂O₅),for example, any aqueous solution of oxalic acid, hydrochloric acid, andsulfuric acid can be used.

After the acid treatment, the NO_(x) removal catalyst 17 a can besubjected to finish water washing, as required, to remove remaining acidfrom the NO_(x) removal catalyst 17 a and is dried.

Vanadium pentoxide (V₂O₅) can be dissolved and removed by using anaqueous solution other than an acidic aqueous solution. This is becausewhen water is used, the sulfur (S) content having deposited in thecatalyst dissolves and becomes an acidic aqueous solution.

The catalyst having been subjected to acid washing or finish washing isdried and is used as a regenerated NO_(x) removal catalyst.

In the present invention, the part 11 a of flue gas having the gastemperature (T₂), which has bypassed the economizer 15 provided in theflue gas duct for the flue gas 16 from the heavy-oil fired boiler 10, issupplied to the upstream of the NO_(x) removal device 17 and mixed withthe flue gas 16 from the economizer 15. The temperature (T₃) of themixed gas 24 is increased to a temperature (360° C. to 450° C.) that ishigher than the temperature (T₁) of flue gas under the normal operatingtemperature condition, and the mixed gas 24 at a high temperature issupplied to the NO_(x) removal device 17 to set the denitrationcondition to a high temperature condition.

Accordingly, during the boiler operation, nitrogen oxide in the flue gas16 can be removed by denitration by ammonia by using the NO_(x) removaldevice 17, and vanadyl sulfate (VOSO₄) adhering to and accumulating onthe NO_(x) removal catalyst 17 a can be decomposed by heating intovanadium pentoxide (V₂O₅).

Sulfur oxide (SO_(x)) such as sulfur dioxide (SO₂) and sulfur trioxide(SO₂) shown in the formula (1) produced at the time of decomposingvanadium can be treated in the existing precipitator 20 and desulfurizer21 in the boiler facility.2VOSO₄→V₂O₅+SO₂+SO₃  (1)

As a result, it is possible to save the time and labor required fortaking out the NO_(x) removal catalyst 17 a to outside and heating itunder a high temperature condition from 450° C. to 600° C. in aseparately provided electric furnace after the boiler operation isfinished, at the time of regenerating the NO_(x) removal catalyst 17 ain a conventional manner.

Further, it can be avoided to perform decomposition of vanadyl sulfate(VOSO₄) under severe conditions as in conventional methods usingsulfuric acid or hydrochloric acid at a high temperature.

When conventional heating treatment is performed in an electric furnace,because the heating treatment is performed in the air, sulfur dioxide(SO₂) and sulfur trioxide (SO₃) hardly exist in processing gas. However,according to the present invention, it is preferable to decrease aproportion of sulfur dioxide (SO₂) and sulfur trioxide (SO₃) containedin the flue gas 16 during combustion in the boiler, in order to promotedecomposition of vanadyl sulfate (VOSO₄) at the time of decomposingvanadyl sulfate (VOSO₄) into vanadium pentoxide (V₂O₅) in the NO_(x)removal device 17.

Therefore, in the pretreatment process during the boiler operation, fuelto be supplied to the boiler needs to be changed from inexpensive heavyoil with a high sulfur (S) content to sulfur (S) fuel having a lowcontent ratio of sulfur (S).

The heavy oil having a high content ratio of sulfur is, for example,ultraheavy oil with a sulfur (S) content of 1% by weight or more, suchas Orimulsion, asphalt, or vacuum residual oil (VRO).

The sulfur (S) fuel having a low content ratio of sulfur (S) used at thetime of changing the fuel at the pretreatment process during the boileroperation is not such ultraheavy oil, but is essentially oil fuel with asulfur (S) content of less than 1% by weight.

More preferably, so-called LSA heavy oil (low sulfur A fuel oil) with asulfur (S) content of 0.5% by weight or less can be used.

This is because when the content of sulfur in flue gas is high, amountsof sulfur dioxide (SO₂) and sulfur trioxide (SO₃) produced by a reactionin the formula (1) decrease, and expensive low-sulfur (S) fuel is usedfor a long time, which is not desirable.

Even if the content of sulfur in oil fuel exceeds 1% by weight, when theSO_(x) concentration in the flue gas 16 before being introduced into theNO_(x) removal device 17 is equal to or lower than 2000 parts permillion, volumetric dry (ppmvd), according to boiler combustionconditions, oil fuel having a content rate of sulfur (S), which isslightly lower than that of normally used low-grade fuel having a highcontent ratio of sulfur, can be used.

This is because when the SO_(x) concentration in the flue gas 16 beforebeing introduced into the NO_(x) removal device 17 exceeds 2000 ppmvd,the concentration of produced SO_(x) is increased and imposes a burdenon an air pollution control facility on a downstream side, which is notdesirable.

When the SO_(x) concentration in the flue gas before being introducedinto the NO_(x) removal device 17 is set to 1000 ppmvd or lower, morepreferably, set to 500 ppmvd or lower, it does not impose a burden onthe air pollution control facility.

By performing the pretreatment before shutdown of the boiler in 1), thetemperature of the mixed gas 24 to be introduced into the NO_(x) removaldevice 17 is increased during the boiler operation, and vanadyl sulfate(VOSO₄) deposited on the NO_(x) removal catalyst 17 a can be decomposedinto vanadium pentoxide (V₂O₅) having solubility in an aqueous solutionof oxalic acid.

Thereafter, the boiler is shut down, the NO_(x) removal catalyst 17 a isextracted and washed with acid by using, for example, an aqueoussolution of oxalic acid, thereby removing vanadium pentoxide (V₂O₅)produced in the NO_(x) removal catalyst.

The acid washing process is a process of removing compounds such asalkali metal (such as Na and K), alkali earth metal (such as Ca and Mg),and vanadium pentoxide (V₂O₅) with its state changed by heatingtreatment from the NO_(x) removal catalyst by dissolving these compoundsin, for example, an aqueous solution of oxalic acid.

At this time, the concentration of oxalic acid in the aqueous solutionof oxalic acid is preferably from 0.5% by weight to 25% by weight (morepreferably, from 4% by weight to 20% by weight). This is because if theconcentration of oxalic acid is lower than 0.5% by weight, the vanadiumcomponent (V₂O₅) and the like cannot be sufficiently removed by washing,and if the concentration thereof exceeds 25% by weight, the costrequired for treatment is increased.

The temperature of the aqueous solution of oxalic acid is preferablyfrom 20° C. to 80° C. This is because if the temperature of the aqueoussolution of oxalic acid is lower than 20° C., vanadium pentoxide (V₂O₅)and the like cannot be sufficiently removed by washing, and if thetemperature thereof is higher than 80° C., the cost required for thetreatment is increased.

When a finish water washing process is performed, oxalic acid adheringto and remaining on the surface of the NO_(x) removal catalyst, alongwith washing using the aqueous solution of oxalic acid, is removed bywashing with water.

At this time, the temperature of the water is preferably from 10° C. to80° C. This is because if the water temperature is lower than 10° C.,adhering residue cannot be sufficiently dissolved in the water andremoved, and if the water temperature is higher than 80° C., it iswasteful in view of thermal energy.

As described above, according to the present invention, when combustionis performed in a boiler by using heavy oil with a high content ofsulfur (S) (1% by weight or more), vanadyl sulfate (VOSO₄) adheres toand accumulates on the surface of the NO_(x) removal catalyst during theoperation of the NO_(x) removal device 17. However, by changing fuelfrom heavy oil to fuel with a low sulfur (S) content (for example, LASheavy oil with a sulfur content of 0.5% by weight or less), and causingthe part 11 a of combustion gas 11 to bypass the economizer 15 by usingthe economizer bypass 15 a and to be mixed with the flue gas 16 from theoutlet of the economizer 15, to generate the mixed gas 24 with thetemperature thereof being increased, vanadyl sulfate (VOSO₄) havingdeposited on the NO_(x) removal catalyst can be decomposed into vanadiumpentoxide (V₂O₅) during the boiler operation.

As a result, it is not required to perform any heating treatment processas in conventional methods, at which after shutdown of the boiler, theextracted NO_(x) removal catalyst is heated in the separately providedelectric furnace, to decompose vanadyl sulfate (VOSO₄) into vanadiumpentoxide (V₂O₅).

Further, an air pollution control facility for an electric furnaceseparately provided for removing sulfur oxide produced in the heatingtreatment is not required, thereby enabling to remove the burden on thefacility cost related to the regeneration treatment.

The NO_(x) removal catalyst to which the regeneration method accordingto the present invention can be applied is not limited to any particulartype, and includes, for example, various NO_(x) removal catalysts suchas a catalyst in which a vanadium-tungsten component is supported ontitania as a carrier, a catalyst in which only a tungsten component issupported on a titania carrier, a catalyst in which avanadium-molybdenum component is supported on a titania carrier, and acatalyst in which an active component other than the componentsmentioned above is supported on a titania carrier.

In addition, a carrier of titania-silica composite oxide can be used asthe carrier.

Furthermore, the regeneration method according to the present inventioncan exhibit the effect mentioned above more remarkably, when the methodis applied to a NO_(x) removal catalyst prepared by formingtitania-tungsten composite oxide, and causing a vanadium-based compoundto be supported on the composite oxide as a carrier, rather than beingapplied to a titanium-tungsten based NO_(x) removal catalyst prepared byforming a titania carrier beforehand, and causing a tungsten componentto be supported on the titania carrier.

The composite oxide type NO_(x) removal catalyst described above can beprepared, for example, by calcining solated metatitanic acid containingat least one compound selected from among tungsten compounds, adding anappropriate amount of an auxiliary agent and water to the resultingcalcination product, or the calcination product mixed with vanadiumoxide, kneading the mixture, extruding the kneaded mixture into alattice form by an extruder, and then drying and calcining the extrudate(see, for example, Japanese Patent Publication No. H1-14808).

More specifically, the composite oxide type NO_(x) removal catalystdescribed above can be easily prepared, for example, by adding apredetermined amount of an aqueous solution of ammonium paratungstate tometatitanic acid, dehydrating, molding, and drying the mixture, and thencalcining the resulting material; or by adding a predetermined amount ofan aqueous solution of ammonium paratungstate to metatitanic acid,dehydrating and drying the mixture, then impregnating the resultingmaterial with an aqueous solution of ammonium metavanadate, molding theimpregnated material, then drying and calcining the molded product.

<Method of Operating Heavy-Oil Fired Boiler>

A method of operating a heavy-oil fired boiler for regenerating a NO_(x)removal catalyst includes the following processes.

a) Normal Operating Process

At the time of a normal operation, the heavy oil F with a high sulfur(S) content is burned in a boiler.

b) Operating Process Before Shutdown

Before shutdown of the boiler, fuel is changed to oil fuel with a lowsulfur (S) content and burned in the boiler in a predetermined period oftime.

In the operating process before shutdown in b), the part 11 a ofcombustion gas having bypassed the economizer 15 provided in the fluegas duct 23 for flue gas from the boiler is supplied to the upstream ofthe NO_(x) removal device 17 and mixed with the flue gas 16 from theeconomizer 15, to generate the mixed gas 24 having a predeterminedtemperature equal to or higher than 360° C. (360° C. to 450° C.), whichis higher than a normal operating temperature condition. The mixed gas24 is introduced into the NO_(x) removal device 17 to have the NO_(x)removal device in a high-temperature denitration condition, so thatnitrogen oxide in the flue gas 16 are removed in the NO_(x) removaldevice 17 during the boiler operation, and vanadyl sulfate (VOSO₄)adhering to and accumulating on the NO_(x) removal catalyst 17 a isdecomposed into vanadium pentoxide (V₂O₅). Furthermore, SO_(x) producedat the time of decomposing vanadium pentoxide (V₂O₅) at the operatingprocess before shutdown can be treated in the desulfurizer 21 of the airpollution control facility on the upstream side of the NO_(x) removaldevice 17.

Shutdown of the boiler includes emergency shutdown in addition to thatfor a periodic inspection.

In the normal operating process in a), the boiler is operated under ahigh temperature condition in which the temperature of the flue gas 16from the economizer 15 is equal to or higher than 400° C., therebyenabling to reduce a produced amount of vanadyl sulfate (VOSO₄) adheringto and accumulating on the NO_(x) removal catalyst 17 a. This is becausethe reaction according to the formula (2) described above does notproceed under the high temperature condition, and accumulation ofvanadyl sulfate (VOSO₄) on the NO_(x) removal catalyst is decreased.

Accordingly, in the normal operating process in a), the produced amountof vanadyl sulfate (VOSO₄) adhering to and accumulating on the NO_(x)removal catalyst 17 a can be reduced by setting a high temperatureoperating condition (operated at a high temperature (T₁=360° C. to 400°C.) higher than the normal operating temperature (T₁=350° C. of the fluegas 16).

As a result, the combustion time for decomposing vanadyl sulfate (VOSO₄)with low S content fuel by using the mixed gas of equal to or lower than450° C., which is mixed with flue gas from the economizer bypass, in theoperating process before shutdown in b) can be reduced.

Consequently, fuel use of the low S content fuel having a fuel unitprice higher than that of heavy oil, which is used at the time ofheating and decomposing vanadyl sulfate (VOSO₄) adhering to andaccumulating on the NO_(x) removal catalyst 17 a, can be reduced.

For example, in the normal operating process, if it is assumed thatoperation hours before shutdown in a case where the boiler is operatedat a temperature of the flue gas 16 from the economizer 15 from 350° C.to 380° C. are, for example, five to seven days, the operation hours canbe reduced to, for example, three to four days by setting thetemperature (T₁) of the flue gas 16 from the economizer 15 equal to orhigher than 400° C. and the temperature (T₃) of the mixed gas mixed withflue gas from the economizer bypass equal to or higher than 450° C.

The operating process before shutdown in b) is identical to the“pretreatment process during the boiler operation” in the method ofregenerating the NO_(x) removal catalyst described above, and thereforedetailed explanations thereof will be omitted.

According to the method of operating the heavy-oil fired boiler of thepresent invention, the pretreatment for regenerating the NO_(x) removalcatalyst can be performed during a boiler operation, and thus hightemperature heating treatment by using a separately provided electricfurnace as in conventional methods is not required, and treatment ofsulfur oxide (SO_(x)) produced at the time of performing the treatmentis not required, thereby enabling to simplify regeneration of the NO_(x)removal catalyst.

Test Example

As a used NO_(x) removal catalyst, a used NO_(x) removal catalyst inwhich vanadyl sulfate (VOSO₄) was deposited on a surface thereof (ahoneycomb catalyst with 6 holes×7 holes×900 millimeters) was prepared.

Simulant gas (with the SO₂ concentration in the gas simulating flue gasof a low sulfur (S) fuel being 500 ppmvd) under the following conditionsin Table 2 was introduced into the used NO_(x) removal catalyst, and SO₃in the gas at the outlet of the catalyst was measured over time, therebyconfirming decomposition of vanadyl sulfate (VOSO₄) in flue gas of thelow sulfur (S) fuel.

X-ray diffraction patterns before and after finishing the test weremeasured.

TABLE 2 Gas condition Gas temperature C. ° 367 Gas flow rate Nm3/hr15.43 SO₂ at Inlet ppmvd 500 SO₃ at Inlet ppmvd 10 O₂ % vd 2 H₂O % v13.2 Ugs Nm/sec 2

FIG. 3 depicts an X-ray diffraction pattern before heating andregeneration. FIG. 4 depicts an X-ray diffraction pattern afterperforming heating treatment with gas containing SO₂ and SO₃.

As shown in FIGS. 3 and 4, in the X-ray diffraction pattern shown inFIG. 4, vanadium pentoxide (V₂O₅), which was a decomposition product ofvanadyl sulfate (VOSO₄), was confirmed, and it was confirmed that evenunder gas conditions containing sulfur oxide, vanadyl sulfate (VOSO₄)was sufficiently decomposed into vanadium pentoxide (V₂O₅).

REFERENCE SIGNS LIST

-   -   10 heavy-oil fired boiler    -   11 combustion gas    -   11 a part of combustion gas    -   12 furnace    -   13 steam drum    -   14 super heater    -   15 economizer    -   15 a economizer bypass    -   16 flue gas    -   17 NO_(x) removal device    -   20 precipitator    -   21 desulfurizer    -   22 purified gas    -   23 flue gas duct    -   24 mixed gas

The invention claimed is:
 1. A method of regenerating a NO_(x) removalcatalyst for removing nitrogen oxide in flue gas at a time of combustionin a boiler by using heavy oil, wherein in a predetermined period oftime before shutdown of a boiler, a decomposing step is performed duringan operation of the boiler, at which a part of combustion gas that hasbypassed an economizer provided in a flue gas duct for flue gas from theboiler is supplied to an upstream of a NO_(x) removal device having aNO_(x) removal catalyst and mixed with flue gas from the economizer, atemperature of mixed gas is increased to a predetermined temperatureequal to or higher than 360° C. to have the NO_(x) removal device in ahigh-temperature denitration condition, so that vanadyl sulfate (VOSO₄)adhering to and accumulating on the NO_(x) removal catalyst isdecomposed into vanadium pentoxide (V₂O₅).
 2. The method of regeneratinga NO_(x) removal catalyst according to claim 1, comprising an acidwashing step after the decomposing step, at which an operation of theboiler is stopped, and thereafter the NO_(x) removal catalyst taken outfrom the NO_(x) removal device is subjected to acid washing.
 3. Themethod of regenerating a NO_(x) removal catalyst according to claim 1,wherein in a boiler operation period at the decomposing step, fuel ischanged to oil fuel with a low sulfur (S) content and is burned in theboiler.
 4. The method of regenerating a NO_(x) removal catalystaccording to claim 2, wherein acid used at the acid washing step is anyaqueous solution of oxalic acid, hydrochloric acid, and sulfuric acid.5. The method of regenerating a NO_(x) removal catalyst according toclaim 2, comprising a water washing step of performing finish waterwashing and a drying step after the acid washing step.
 6. The method ofregenerating a NO_(x) removal catalyst according to claim 3, wherein oilfuel with a low sulfur (S) content is oil fuel that contains 1% byweight or less of sulfur (S).
 7. The method of regenerating a NO_(x)removal catalyst according to claim 2, wherein in a boiler operationperiod at the decomposing step, fuel is changed to oil fuel with a lowsulfur (S) content and is burned in the boiler.
 8. The method ofregenerating a NO_(x) removal catalyst according to claim 4, comprisinga water washing step of performing finish water washing and a dryingstep after the acid washing step.